1. Field of the Invention
The present invention relates to the field of automobile wheels and more particularly to an improved automobile wheel rim and method of making same.
2. Description of the Prior Art
Automobile owners often desire to improve the appearance and performance of their cars by replacing the original equipment wheels supplied by the vehicle manufacturer (OEM) with aftermarket wheels of lighter weight and more stylish appearance. While many owners are satisfied with aftermarket wheels that are comparatively more stylish than OEM wheels, some owners, "automotive enthusiasts" (drivers of sports and high performance type cars), desire wheels that are of substantially lighter weight and therefore improve the performance as well as the appearance of their vehicles.
Automobile wheels of a type which provide a decorative appearance and relatively light weight are most commonly manufactured by using a light weight casting alloy, typically A356 or A357 aluminum, to cast a wheel in the desired configuration. Typically these wheels are cast as one piece units with integral wheel-centers and rims. After casting, the wheels are finish machined to final dimensions. There are many examples of wheels of this type, one of which is U.S. Pat. No. 5,292,182, entitled "Light Alloy Cast Wheel" issued to Kanazawa et al, Mar. 8, 1994. Some cast alloy wheels possess an ornamental design feature that provides for a smooth, stepless, radially outwardly curving relatively thick appearing outboard flange. This appearance feature has proved to be popular in the aftermarket. An example of a wheel of this type is U.S. Pat. No. Des. 389,446, entitled "Rim for a Vehicle Wheel" issued to Bradley, Jan. 20, 1998.
That is, cast alloy wheels though economical to manufacture and appropriately decorative, possess certain drawbacks that render them undesirable to "automotive enthusiasts" who desire wheels of lighter weight and greater strength. The major drawback of a cast alloy wheel is excess weight. The most common alloys (A356 and A357) used in the production of these wheels possess relatively low yield and ultimate tensile strengths. The casting process produces a metal structure that is porous and brittle in nature, therefore resulting in a lower fatigue life in comparison to cold rolled alloy sheet of equivalent thickness. To account for these deficiencies manufacturers of cast alloy wheels must use a relatively large mass of material and thereby produce a wheel that is heavier than that produced by other methods. Quality control is also a problem in the manufacture of cast wheels as differential cooling between relatively thick and comparatively thinner sections of an integral one piece wheel often results in the formation of voids and blowholes in the finished product.
Another method of producing automotive wheels is cold forging. Due to their high cost, cold forged wheels are rarely seen on passenger cars but, rather, are used on high performance racing vehicles almost exclusively. Cold forged wheels are manufactured from cold rolled aluminum alloy tube, bar, or plate stock. Cold rolled aluminum alloy has a dense grain structure giving the metal excellent ductility and a high fatigue strength. Alloys commonly used in forging such as 7075 and 7050 series aluminum have significantly higher yield and tensile strength than the A356 and A357 casting alloys. A cold forged wheel may be manufactured as a one or two piece unit. The process begins by hammer forging the initial stock (bar, tube, or plate) into a donut shape. The rim portion is coaxially extruded from the forged blank. In the case of a one piece unit, after extrusion of the rim, the hub is forged radially inwardly to its desired configuration. Critical surfaces are then finish machined. An example of a cold forged wheel is described in U.S. Pat. No. 5,446,962, entitled "Process of Manufacturing One Piece Forged Wheels" issued to Matossian et al, Sep. 5, 1995. Cold forged wheels have the characteristics of light weight and high strength that arc greatly prized among "automotive enthusiasts". However, due to the high cost of producing these wheels few enthusiasts can aspire to owning a set. Thus, there remains a need for a lightweight, high strength wheel having, as an ornamental design feature, an outboard flange that flares radially outwardly in an attractive manner and exhibits an appearance of having axial depth, that can be produced at a cost comparable to that of a cast wheel.